1. Field of the Invention
This invention relates generally to testing vision, in particular to testing contrast sensitivity.
2. Background
A number of qualities or functions are used to describe how good a person's eyesight is. One such function is “visual acuity,” which can be described as the ability to resolve or distinguish details of objects, as determined by the smallest object that a person can identify at a specified distance. Visual acuity can be measured by what many people think of as the most common eye test: reading with one eye at a time a chart having several lines of letters, typically with one large letter (such as a capital “E”) on the top line, and additional lines having an increasing number of letters that decrease in size. Normal visual acuity is generally referred to as 20/20 vision, meaning that a particular person sees clearly at 20 feet what humans can normally see clearly at 20 feet. Someone with 20/40 vision can see clearly at 20 feet what people normally can see clearly at 40 feet.
Another aspect of eyesight or vision is the “visual field” or “field of vision,” which can be described as where a person can see, or the full extent of the area visible to an eye that is fixed, or fixating, straight ahead. Visual field is usually expressed as angles measured in degrees from a point of fixation. Horizontally, normal visual field from a point of fixation is about 95 degrees toward a person's temple and about 60 degrees toward a person's nose; vertically, normal visual field is about 50 degrees above and 65 degrees below the point of fixation.
Yet another visual function is “contrast sensitivity,” which can be described as the ability to see a difference in contrast between an image and the background of an image. For example, black writing on a white billboard normally presents a high contrast that someone with normal vision can see easily. On a very foggy day, the black writing and the white background can appear grayish and be more difficult to see. Someone with good contrast sensitivity might be able to see the writing on the billboard well enough to read it through the fog, while someone else with poor contrast sensitivity may be unable to do so.
In addition, contrast sensitivity is fundamental to visual acuity and visual field. Thus, contrast sensitivity is increasingly recognized as an important, perhaps the most important, visual function in determining how well individuals can perform the activities of daily living such as reading, doing housework, watching television, and getting around from place to place. This makes tests for contrast sensitivity increasingly important.
But previously known methods of testing contrast sensitivity exhibit several disadvantages. Tests in which a person is asked to read letters (e.g., the Pelli Robson test) depend upon the person's ability to resolve objects—that is, the person's visual acuity. For individuals with poor visual acuity, such tests have a decreased ability to determine contrast sensitivity. For example, certain eye diseases, such as macular degeneration, affect the central portion of the eye's retina, which is where resolution is highest and is used in activities such as reading. Thus, for someone afflicted with such an eye disease, a test for contrast sensitivity that includes reading letters will be confounded by relying heavily on resolution (i.e., visual acuity), thus providing an inaccurate measure of contrast sensitivity. In addition, such a contrast sensitivity test may be difficult or impossible to administer to illiterate persons.
Some other previously known contrast sensitivity tests ask a person to stare at a fixed point while small bursts of light (or similar illumination or animation) appear in the person's peripheral vision. For example, a person may be asked to stare at a fixed point on a computer screen while small, animated “bursts” are displayed at various peripheral points on the screen. But such contrast sensitivity test tests depend on the person's visual field: for individuals with constricted visual fields, such tests have a decreased ability to determine contrast sensitivity. For example, certain eye diseases, such as glaucoma, can affect peripheral vision. Therefore, for someone afflicted with such an eye disease, a test for contrast sensitivity that includes peripheral bursts of illumination will be confounded by relying heavily on visible area (i.e., visual field), again providing an inaccurate measure of contrast sensitivity.